TCP/IP is widely used as a connection-oriented communication protocol between communication terminals. In the case of using TCP/IP in a network with high latency, congestion control of TCP/IP causes a decrease in throughput. The TCP/IP communication throughput depends on an RTT (Round Trip Time), which is latency in a network between a sender and a receiver, and a packet loss rate. The TCP/IP communication throughput is substantially inversely proportional to the RTT and inversely proportional to the square root of the packet loss rate. In detail, the TCP/IP communication throughput is expressed by the following model equation described in Non Patent Literature (NPL) 1.
                              [                      Math            .                                                  ⁢            1                    ]                ⁢                                                                                                B          ⁡                      (            p            )                          ≈                  min          (                                                    W                ⁢                                                                  ⁢                max                            RTT                        ,                          1                                                                                                                  RTT                        ⁢                                                                                                            2                              ⁢                              bp                                                        3                                                                                              +                                                                                                                                                          T                        0                                            ⁢                                              min                        (                                                  1                          ,                          3                          ⁢                                                                                                                    3                                ⁢                                bp                                                            8                                                                                                      )                                            ⁢                                              p                        ⁡                                                  (                                                      1                            +                                                          32                              ⁢                                                              p                                2                                                                                                              )                                                                                                                                                  )                                    Equation        ⁢                                  ⁢                  (          1          )                    
In Equation (1), “B” denotes the number of packets per unit time, “W” denotes the maximum window size (typically about 8 to 200 packets) in TCP, “RTT” denotes the RTT when a packet travels from one terminal to the other terminal and back again, “p” denotes the packet loss rate, “T0” denotes the retransmission timeout period (typically 1 second), and “b” denotes the number of ACKs (typically 2) combined according to TCP's delayed ACK option.
To improve the TCP/IP communication throughput, window control algorithms such as TCP Tahoe, TCP Reno, and TCP SACK are proposed. Window control algorithms for long-distance high-speed transmission, such as High Speed TCP, are proposed, too.
In a long-distance system such as a submarine cable system, a signal is transmitted with a strong error correction code such as FEC (Forward Error Correction) being added thereto. Accordingly, when an appropriate transmission path is designed, code errors are mostly negligible. However, transmission path delays due to optical fibers or repeaters can incur high latency and decrease the TCP/IP communication throughput.
If a physical interface is low-speed, the decrease of the TCP/IP communication throughput caused by latency is not apparent in the transmission path. As a result of significant improvements in physical interface speed, however, the decrease of the TCP/IP communication throughput forms a bottleneck in the transmission path. In a long-distance system such as a submarine cable system, too, the decrease of the TCP/IP throughput caused by latency in the transmission path can be a bottleneck.
Patent Literature (PTL) 1 describes a system for improving the TCP/IP communication throughput by performing protocol conversion to separate a protocol in a submarine cable segment and a protocol in a land transmission network submarine cable segment 601.
PTL 2 describes a device capable of realizing quick path switching without overhead or processing delay, reconstructing an alternate path to bypass a specific congested link, and relaying traffic in a distributed manner in an entire network.